In traction control operation, control of wheel slip on certain road surfaces may interact with natural frequencies in the driveline or suspension, which may be referred to as power induced “hop”. Hop resonances are caused by interactions between tire, road, and suspension components.
On typical road surfaces, the transfer of engine power into vehicle acceleration and frictional losses at the tire keeps the amplitude of the hop resonance relatively low. However, the hop resonance may increase to a troublesome level on certain road surfaces. For example, on an asphalt surface with a thin layer of standing water, the friction coefficient (μ) and surface torque capacity may change significantly with respect to wheel slip and suspension loading. When a tire has full suspension force acting on it, the tire-road frictional torque capacity may be at a desired value for normal traction control and vehicle operation. As the suspension unloads the tire (for various reasons including surface roughness), water may be present between the road surface and tire, causing a loss of frictional torque capacity. Because the suspension may quickly unload the tire in the fore/aft direction, the thickness of the water layer between the tire and road may be altered. As a result, the tire-road frictional torque capacity may be varied. This cyclical interaction may excite the normally insignificant suspension vibration or hop resonance, and initiate significant oscillatory wheel acceleration, causing the wheels, or even the entire vehicle, to bounce. Without mitigation, the traction control system may further excite the vehicle system by way of in-phase powertrain torque response to the oscillating wheel acceleration. Excitation of hop may correspond to an increase in vehicle noise, vibration, and harshness (NHV) and a potential durability concern.
One solution to control the power-induced hop may include stopping the unwanted acceleration and deceleration by applying counter-phase torque. However, the implementation of such actions may require a fast hop detecting or computing device and/or well-tuned, fast-acting, actuating components. Consequently, it may be cost prohibitive for mass produced vehicles, such as those using traction control via engine torque reduction without brake actuation.